Bump-on-Trace (BOT) structures are used in flip chip packages, wherein metal bumps are bonded onto narrow metal traces in package substrates directly, rather than bonded onto metal pads that have larger sizes than the respective connecting metal traces. The BOT structures require smaller chip areas, and the manufacturing cost of the BOT structures is low. The conventional BOT structures may achieve the same reliability as the conventional bond structures that are based on metal pads. In a typical BOT structure, a solder region is formed on a surface of a copper bump of a device die. The solder region bonds the copper bump to a metal trace in a package substrate. The solder region contacts a top surface and sidewalls of the metal trace, hence forming the BOT structure.
Since the existing BOT structures have very small spacings, bridging may occur, wherein the solder region of one BOT bond structure is bridged to a neighboring metal trace. Particularly, the BOT structures in the peripheral areas of the packages are more likely to bridge due to the high density of the BOT structures in the peripheral areas. In addition, in the peripheral areas, the distance of the BOT structures are farther away from the centers of the respective packages. Accordingly, during the reflow process for forming the BOT structures, the shift of the BOT structures caused by the thermal expansion of the metal traces is more significant than in the areas close to the centers of the respective packages. Accordingly, the bridging is more likely to occur.
Previously, to reduce the likelihood of the bridging in BOT structures, either narrow metal traces are used, or less solder is used. When the metal traces are narrowed to reduce the bridging, since the adhesion of the metal traces to the respective underlying dielectric layer is related to the contacting area between the metal traces and the dielectric layer, with the reduction in the metal traces, the contacting area is reduced, the adhesion between the metal traces and the dielectric layer degrades. As a result, metal traces are more likely to peel off from the dielectric layer. On the other hand, if less solder is used to reduce the bridging, the stress that occur to the solder region is applied on a small solder region, and solder crack is more likely to occur than on a larger solder region.